“My principal concern is that if you spend this money on this one railway line then we will not have the money on maintaining and upgrading existing lines,” Mr Darling said.

He added: “If you do not spend money on upgrading and improving the track and the trains, then eventually things will start falling apart, as they did in the mid 1990s. And that would be catastrophic.”

Patrick McLoughlin, the Tory Transport Secretary, was yesterday forced to insist that HS2 still has the backing of the Prime Minister and George Osborne, the Chancellor.

“The Chancellor and the Prime Minister have been totally in support of this project and there is no problem with support that I'm receiving from other colleagues in Government,” he told BBC Radio 4’s World at One programme.

Work on HS2 is due to begin in 2017, with connections to Manchester and Leeds completed by 2032.

Experts have warned that HS2’s economic case does not stack up and that it will blight large tracts of the British countryside.

Mr McLoughlin added: “This scheme is very important to the infrastructure of this country and all big infrastructure projects are always controversial. No doubt Alistair Darling knew that when he signed it off as Chancellor of the Exchequer.”

A number of Tory MPs are bitterly opposed to the scheme, with former Welsh Secretary Cheryl Gillan describing the project as a “living nightmare” for thousands of constituents along the route.

Mrs Gillan said that continuing with the HS2 project “defies reason”.

“It is not just my constituents, who are already feeling the impacts, but every taxpayer in the country who will lose out at the hands of this project,” she said.

“To continue pushing ahead with HS2 simply defies reason and shows a total disregard for the consequences. Given the weight of evidence against it, showing the escalating costs and the lack of a comprehensive and credible business case, the time has come to cancel HS2.”

Writing on Twitter Lord Ashcroft said that it was now “probably also inevitable that the Labour Party will come out against HS2”.

However a Labour spokesman said: “A new North-south rail line is vital to deliver the major increase in capacity needed in Britain's rail network in years to come.

“It will significantly reduce journey times, between our towns and cities, it will create desperately needed jobs, growth, regeneration and rebalance the economy between London and the rest of Britain.”